Eugene Chekaluk

Measuring Visual Discomfort

A two-parameter Rasch Rating Scale model was developed to measure visual discomfort. Initially it was found that participants reporting frequent severe headache, reading difficulties of a visual nature, and short effective reading times experienced more severe visual discomfort. The validity of this measurement instrument was tested in four experiments. In Experiments 1 and 2 reports of unpleasant somatic and perceptual side-effects or ratings of unpleasantness were obtained for low, moderate, and high scorers on the Visual Discomfort Scale. It was found that those with higher scores reported a greater number of unpleasant side-effects and rated square-wave patterns across the spatial frequency range, and a letter pattern as more unpleasant and distorted to view than those with low scores on the scale. In Experiments 3 and 4 these subjective findings were extended to performance. Efficiency was measured using a copying and reading task. It was found that those obtaining high scores on the Visual Discomfor...

Visual stimulus input, saccadic suppression, and detection of information from the postsaccade scene

Tachistoscopic presentation of saccadic stimulus sequences to fixating subjects produced saccadic suppression curves only when the pre- and postsaccade fixation fields were structured. Displacement in the sense either of movement of the intrasaccade display or of change from pre-to postsaccade fixation field was not required. Variation of intrasaccade displays from contours moving at saccadic rates to a stationary gray field had no effect. When one structured field immediately followed another, the change of sensitivity mimicked suppression, but an interposed grayout magnified the loss and therefore probably reduced carryover from the first fixation field. Interposed grayout also delayed recovery but reduced the latency of detection of information from the postfield, so that it appears to have reduced stimulus overload from the sudden presentation of the second field.

The Effects of Visual Discomfort and Pattern Structure on Visual Search

Unpleasant somatic and perceptual side effects can be induced when viewing striped repetitive patterns, such as a square wave or a page of text. This sensitivity is greater in participants with higher scores on a scale of visual discomfort. In three experiments the effect that this sensitivity has on performance efficiency in a reading-like visual search task was investigated. In experiments 1 and 2, the ‘global’ structure of the patterns was manipulated to produce a square-wave, a checkerboard, and a plaid pattern. It was found that the group that suffered severe visual discomfort took significantly longer than other groups to perform the task, with interference greatest with presentation of the square-wave-like pattern. This supports the prediction of greatest distraction of visual attention from the local target elements with presentation of the pattern structure inducing greatest visual discomfort. In experiment 3, the internal pattern components were manipulated and task difficulty reduced. A no-interference and two interference patterns, one with a global characteristic only and the second made up of distracting line elements, containing global and local components were used. The global pattern structure produced interference effects on the visual-search task. All groups performed with the same speed and accuracy on the task involving the no-interference pattern, a finding attributed to reduced task difficulty McConkie and Zolas model of visual attention was used to explain these results.

Visual discomfort: The influence of spatial frequency

The response of different visual discomfort groups to a range of spatial frequencies at threshold and suprathreshold was investigated. In experiment 1, a paired-comparison task was conducted. The high visual discomfort group judged a spatial frequency of 4 cycles deg−1 as the most perceptually distorted and somatically unpleasant to view. The moderate and low visual discomfort groups judged 8 and 12 cycles deg−1 as more perceptually and somatically unpleasant to view than lower spatial frequencies. In experiment 2, the spatial contrast-sensitivity function (CSF) for the high visual discomfort group was depressed for spatial frequencies between 1 and 12 cycles deg−1 in comparison with the moderate and low visual discomfort groups. When these same spatial frequencies were modulated at 6 Hz, CSFs were the same for all groups. These results are discussed in relation to a failure of inhibition across spatial-frequency channels in the high visual discomfort group. This may be explained by a more generalised parvocellular system processing deficit. Possible similarities between some forms of migraine and visual discomfort are highlighted.

Cognitive Tests and Determining Fitness to Drive in Dementia: A Systematic Review

Evidence has shown that although all individuals with dementia will eventually need to stop driving, most can continue to drive safely early in the disease. Fitness to drive needs to be monitored, and the use of cognitive testing to determine driver safety has been suggested. This review is the first to examine cognitive test results pertaining only to individuals with dementia. The aim was to examine the relationship between cognitive tests and driving to determine whether a cognitive assessment can be implemented as a tool to examine driver safety. A systematic review of 28 studies investigating the relationship between cognitive functioning and driving in individuals with dementia was conducted. The results of this review demonstrated a lack of consistency in the findings, with some studies showing a relationship between cognitive testing and driving performance for individuals with dementia, whereas others did not. Results relating to individual cognitive tests and measures confined to a single cognitive domain were variable and not consistently associated with driving performance. Studies consistently found that composite batteries predicted driving performance. The findings from this review support the use of composite batteries comprising multiple individual tests from different cognitive domains in predicting driving performance for individuals with dementia. Scores on individual tests or tests of a single cognitive domain did not predict driver safety. The composite batteries that researchers have examined are not clinically usable because they lack the ability to discriminate sufficiently between safe and unsafe drivers. Researchers need to develop a reliable, valid composite battery that can correctly determine driver safety in individuals with dementia.

A Comparison of the Degree of Effort Involved in the TOMM and the ACS Word Choice Test Using a Dual-Task Paradigm

The aims of the current study were to: (a) examine the predictive validity and efficacy of the Advanced Clinical Solutions Word Choice Test (WCT) as a measure of effort relative to the Test of Memory Malingering (TOMM); (b) investigate whether performing a dual (distraction) task would undermine performance on either test; (c) assess the effect of coaching on the diagnostic accuracy of both the WCT and the TOMM; and (d) establish an optimal cut score for the WCT. The current study used a simulation design based on an analogue design in which normal participants were instructed to either apply full effort or simulate a brain injury on the tasks without being detected. Participants included 93 undergraduate university students who were randomly assigned to 1 of 4 conditions: (a) distraction, (b) uncoached traumatic brain injury (TBI) simulators, (c) coached TBI simulators, or (d) full effort. The results demonstrated that the WCT and the TOMM were effective in detecting simulated cognitive impairment. Both tests were resistant to the effects of distraction and were equally effective in detecting coached and uncoached simulators. A cut score of 42 on the WCT was found to provide optimal specificity and sensitivity on the test.

Does a sensory processing deficit explain counting accuracy on rapid visual sequencing tasks in adults with and without dyslexia

The experiments conducted aimed to investigate whether reduced accuracy when counting stimuli presented in rapid temporal sequence in adults with dyslexia could be explained by a sensory processing deficit, a general slowing in processing speed or difficulties shifting attention between stimuli. To achieve these aims, the influence of the inter-stimulus interval (ISI), stimulus duration, and sequence length were evaluated in two experiments. In the first that used skilled readers only, significantly more errors were found with presentation of long sequences when the ISI or stimulus durations were short. Experiment 2 used a wider range of ISIs and stimulus durations. Compared to skilled readers, a group with dyslexia had reduced accuracy on two-stimulus sequences when the ISI was short, but not when the ISI was long. Although reduced accuracy was found on all short and long sequences by the group with dyslexia, when performance on two-stimulus sequences was used as an index of sensory processing efficiency and controlled, group differences were found with presentation of stimuli of short duration only. We concluded that continuous, repetitive stimulation to the same visual area can produce a capacity limitation on rapid counting tasks in all readers when the ISIs or stimulus durations are short. While reduced accuracy on rapid sequential counting tasks can be explained by a sensory processing deficit when the stimulus duration is long, slower processing speed in the group with dyslexia explains the greater inaccuracy found as sequence length is increased when the stimulus duration is short.

Saccadic Suppression: A Functional Viewpoint

Publisher Summary This chapter discusses a functional viewpoint on saccadic suppression. It is attempted to provide sufficient explanatory data to account for saccadic suppression under ordinary viewing conditions. These are those conditions of a contour-rich visual environment in photopic presentation. The chapter focuses on the factors that can lead to a drop in visual sensitivity when saccades are made under ordinary viewing conditions. A variety of techniques used to create saccade analogues is compared to real saccades and visual sensitivity, among other variables, measured. In all cases, the sensitivity function obtained psychophysically has mimicked that obtained with the saccade analogue. The suggestion is therefore that the eye movement itself or the neural mechanism responsible for its initiation is not necessary to explain visual sensitivity loss during saccades. In addition, the chapter extends those findings to investigations in both the saccadic suppression tradition where a probe target is used and in the saccadic omission tradition where the gray-out itself is considered the effective visual stimulus.

Impaired Driving Performance as Evidence of a Magnocellular Deficit in Dyslexia and Visual Stress.

High comorbidity and an overlap in symptomology have been demonstrated between dyslexia and visual stress. Several researchers have hypothesized an underlying or causal influence that may account for this relationship. The magnocellular theory of dyslexia proposes that a deficit in visuo-temporal processing can explain symptomology for both disorders. If the magnocellular theory holds true, individuals who experience symptomology for these disorders should show impairment on a visuo-temporal task, such as driving. Eighteen male participants formed the sample for this study. Self-report measures assessed dyslexia and visual stress symptomology as well as participant IQ. Participants completed a drive simulation in which errors in response to road signs were measured. Bivariate correlations revealed significant associations between scores on measures of dyslexia and visual stress. Results also demonstrated that self-reported symptomology predicts magnocellular impairment as measured by performance on a driving task. Results from this study suggest that a magnocellular deficit offers a likely explanation for individuals who report high symptomology across both conditions. While conclusions about the impact of these disorders on driving performance should not be derived from this research alone, this study provides a platform for the development of future research, utilizing a clinical population and on-road driving assessment techniques.

Masking effects in saccadic eye movements

Saccadic eye movements made under normal viewing conditions are accompanied by a loss of visual sensitivity commonly labelled saccadic suppression or saccadic omission. Most explanations of the sensitivity loss that involve retinal factors imply visual masking. Despite this, the range of masking effects which may be involved here have not received much attention, with most research focusing on metacontrast effects. The experimental work presented here forms the first steps towards fully analyzing masking effects which may be involved in saccadic eye movements, and shows that metacontrast is only one type of masking that plays a role.